It is known in the art to provide a pressure modulating valve in the pressure fluid line of a vehicle brake system between a master cylinder and at least one of the rear wheel brake cylinders.
A pressure modulating valve interposed in a brake system functions to compensate for difference in the effectiveness of the front and rear brakes. The differences in effectiveness is due to the dynamic weight shift to the front axle during a brake application. It is the weight shift which produces the principle need for a pressure modulating valve.
The imbalance between the effectiveness of the front and rear brakes is particularly apparent in a truck and varies substantially depending upon the amount of load which the truck carries. In a fully loaded truck, the weight on the rear axle may be such that the rear brakes have braking capability that is almost as great as that of the front wheels. When the same truck is in an unladen condition, however, its rear brakes will not be nearly as effective as the front brakes. In such an unladen truck, equal pressure at the front and rear brakes would produce rear wheel skid when the master cylinder pressure is high enough to produce a moderate rate of vehicle deceleration.
In a truck, where the relative effectiveness of the front and rear brakes changes greately depending upon the load on the vehicle, it is desirable to have a modulating valve with a changeover point that may be modulated or regulated in response to the amount of load and the effectiveness of the brakes.
A known modulating valve embodying the desired effect employs an inertia ball which, when the vehicle deceleration reaches a fixed value, rolls up an inclined ramp to seat against a valve seat, isolating the rear brake cylinders, and any further increase in master cylinder pressure after changeover point or pressure is prevented from being transmitted to the rear brake cylinders or is transmitted to some extent to the rear brake cylinders. The inertia ball can modulate the changeover pressure in response to the amount of load carried by the vehicle because assuming that the vehicle is heavity loaded, the required magnitude of deceleration at which the inertia ball seats against the valve seat will be achieved when a relatively high master cylinder pressure exists. However, the fact that the required magnitude of deceleration at which the inertia ball seats against the valve seat is fixed causes a problem that if, in designing a brake system, the required magnitude at which the inertia ball seats against the valve seat is so adjusted as to provide the braking ratio approaching the ideal braking ratio when the vehicle is unladen, the changeover pressure which will be obtained when the same vehicle is fully laden will be so low that the braking ratio obtained deviates considerably from the ideal braking ratio when the vehicle is fully laden, thereby increasing the stopping distance during braking operation under this condition.